Similar to the phrase 'up the creek without a paddle'; it is used to describe a terrible situation which there is no way to resolve. 'Horder' is a known bummer, so to be round his house without an arse-guard suggest an anal-rape is likely for the guardless individual.

I acidentally flushed my friends english coursework down the the toliet the day before he had to hand it in. If he finds out I'm round Horder's without an arse-guard.